Gary,
you need a water meter then, not flow meter. flow meter only tell you the rate of water flow such as GPM. btw, you don't need to purchase the water meter to do what you want to achieve.
before you fill your new pond up, turn off every water faucets in your home and ask every to stop use water including flush the toilets. open up your home water meter on the side walk and write down its current reading. find out how your water meter work in term of reading the usage. my meter reading is 100 cubic foot per digit so for example.
current reading of 123456
after filled the pond reading 123459
diffs = 4 => 400 cubic foot
1 cubit foot = 7.48 gallons
so my pond would have 2992 gals.
I normally stop 2" before reaching the water line where I wanted to fill the pond up to. then take the water reading there before fill up another inch of water. this way, I can calculate and find out how many gals per inch of water in my pond. this is helpful when I change the water. I dump the water until it drop to the 1" mark or to 2" mark and I know exactly how many gals I need to refill.
you only need to do this once and keep the record. when you treat your pond with meds, you know exactly how many gals you have in the pond.
Steve